The Fisher Body Unit Number is unique to Fisher Body meaning it has nothing
to do the car's final VIN and will not match the last six digits of the
car's VIN.

Unlike the VIN that increases by 1 for each car regardless of the series
or body style the body number appears to have no association
with the time frame of the build or a particular series or model. This means that any two Chevelles built one after the other
would have consecutive VINs such as 101000 and 101001 but the body
numbers would not be consecutive or even close to each other.